Method and system for detecting and classifying the modulation of unknown analog and digital telecommunications signals

A method for recognizing the type of modulation of a complex baseband signal extracted from a modulated signal having a carrier frequency, said method comprising the steps of:

• (a) detecting presence of the baseband signal by estimating the power spectral density thereof with the aid of externally measured background noise power, and comparing said power spectral density against a power threshold derived from the background noise and a predefined probability of detection;
• (b) estimating baseband frequency bandwidth by comparing the power spectral density against a bandwidth threshold derived from the background noise and a predefined bandwidth estimation error and a predefined probability of detection;
• (c) estimating main centroid frequency of the baseband signal, by computing a component centroid frequency of at least one frequency component in the power spectral density bearing a power above the bandwidth threshold;
• (d) performing gross carrier error correction by frequency translating the baseband signal by the main centroid frequency, followed by low-pass filtering with a filter bandwidth derived from the baseband frequency bandwidth, thereby generating a pre-processed signal from the baseband signal;
• (e) examining amplitude variations in the pre-processed signal to identify the type of envelope thereof as being one of a constant envelope and one of an irregular envelope;
• (f) estimating the carrier frequency error in the pre-processed signal by performing the steps of:
• (i) obtaining the normalized squared amplitude of the carrier-corrected signal and applying thereto a Fast Fourier Transform with zero-padding to generate an FFT output;
• (ii) squaring the absolute values of the FFT output to generate a frequency spectrum;
• (iii) searching the frequency spectrum to find a maximum-power frequency sample;
• and
• (iv) performing a fine search for the carrier frequency error, by applying one step of a secant optimization process to the maximum-power frequency sample and the pre-processed signal;
• when the type of envelope is identified as a constant envelope;
• and
• by performing the steps of:
• (v) obtaining the normalized squared amplitude of the carrier-corrected signal and applying thereto a Fast Fourier Transform with zero-padding to generate an FFT output;
• (vi) obtaining the square of the absolute values of the FFT output to generate a frequency spectrum;
• (vii) searching the frequency spectrum to find a maximum-power frequency sample;
• (viii) performing a fine search for the carrier frequency error, by applying one step of a secant optimization process to the maximum-power frequency sample and the output of the normalized squared amplitude signal,
• when the type of envelope is identified as an irregular envelope;
• (g) correcting the pre-processed signal for the carrier frequency errors to generate a carrier-corrected signal;
• and
• (h) classifying the type of modulation in the carrier-corrected signal, based on the type of envelope,
• by performing the steps of:
• (i) obtaining the direct phase variance for samples of the carrier-corrected signal above the mean amplitude of the carrier-corrected signal;
• (ii) comparing the direct phase variance to a predefined phase threshold;
• and
• (iii) classifying the type of modulation as Continuous Wave when the direct phase variance is below the phase threshold, and as Frequency Modulation when the direct phase variance is above the phase threshold,
• when the type of envelope is identified as a constant envelope, and by performing the step of classifying the type of modulation includes a step of obtaining from the carrier-corrected signal the variance of the absolute phase φa(t)=∠(|I(t)|+j|Q(t)|) for signal samples above the mean signal amplitude, to classify the type of modulation when the absolute phase variance is below a predefined phase threshold as being one of Amplitude Modulation, Double Sideband Suppressed Carrier, and Binary Phase Shift Keying, and, when the absolute phase variance is above a pre-defined phase threshold, as being one of Quarternary Phase Shift Keying (QPSK), π/4-QPSK, M-ary PSK, and OTHER, when the type of envelope is identified as an irregular envelope.

A system for recognizing the type of modulation of a modulated signal having a carrier frequency comprising:

• (a) a receiver section for extracting from the modulated signal a complex baseband signal;
• (b) a pre-classification stage for generating a pre-processed signal from the baseband signal;
• (c) means for examining amplitude variations in the pre-processed signal to identify the type of envelope thereof as being one of a constant envelope and an irregular envelope;
• (d) means for estimating the carrier frequency error in the pre-processed signal;
• (e) means for correcting the pre-processed signal for the carrier frequency errors to generate a carrier-corrected signal;
• and
• (f) means for classifying the type of modulation in the carrier-corrected signal, based on the type of envelope identified in the examining step.